The present invention relates to a system and method for predistorting a signal prior to input to an amplifier in order to cancel out memory components introduced prior to input to the amplifier by a filtering effect and, more particularly, to equalization of a postamplifier signal for use only in the predistortion of transmitted signals.
Transmitters used in high data rate communication links, such as in certain satellite communications systems typically employ high power amplifiers (HPAs), such as traveling wavetube amplifiers (TWTAs) and solid state power amplifiers (SSPAs),. These types of high speed communication systems typically need a relatively high output power so that the signal being transmitted can travel greater distances before being significantly attenuated. In these types of communication systems, a low frequency digital baseband signal comprising a stream of digital data bits is transmitted after modulation onto a high frequency carrier wave.
Different modulation schemes in the art distinguish the digital bits. Example digital modulation schemes for different applications include amplitude-shift keying (ASK), frequency-shift keying (FSK), binary phase-shift keying (BPSK), quadrature-phase shift keying (QPSK), and quadrature amplitude modulation (QAM). Also, the digital baseband signals may be multilevel (M-ary) signals requiring multilevel modulation methods. Quadrature modulation schemes provide both amplitude and phase modulation of the carrier because both complex and imaginary representations of the signal are used.
In quadrature modulation schemes, such as QAM, each bit is converted to a bit symbol representing a complex value having an in-phase (real) component and a quadrature-phase (imaginary) component. A constellation pattern represents a group of symbols positioned within a circle around the origin of an imaginary axis and a real axis. The distance from the origin represents the amount of power being transmitted. For example, a group of four bits transmitted at a particular time is represented as sixteen (24) symbols in the circle. Each symbol in the pattern identifies a complex voltage value having an in-phase component and a quadrature-phase component and represents the voltage value for a particular symbol period, which is the time during which each symbol is transmitted. The analog voltage value for each symbol is used to modulate a carrier wave. The symbols in the constellation pattern are geometrically spread so that they are equally spaced apart to more readily distinguish the symbols and reduce bit errors and may be positioned on one or more circles centered about the origin of the constellation pattern. Preferably, the constellation patterns get processed through the transmitter without being distorted so that the bits are readily distinguishable from each other at the receiver end.
High power amplifiers (HPAs) are desirable in high speed communication applications because they provide high gain over wide bandwidths. However, the input signals to a HPA must be controlled because the HPA exhibits non-linear transfer characteristics. At lower input powers, the output-input power relationship of the HPA is approximately linear. However, at peak power output, the HPA saturates and further increases in the input power beyond the saturation point actually decrease the output power of the amplifier.
The non-linearity of the HPA affects the position of the symbols in the constellation pattern by moving them away from the origin. Therefore, it is known to provide amplifier predistortion techniques in the transmitter when the amplifier is being operated in its non-linear range near peak output power. This predistortion approach typically includes using a memoryless mapping function that employs look-up tables that preset the constellation pattern symbols closer to the origin, so that when the signal passes through the amplifier, the symbols are moved towards locations representative of a linear transfer function.
High power amplifiers also include filtering distortions that cause the amplifier to have memory of previous constellation symbols already transmitted. The term xe2x80x9camplifier memoryxe2x80x9d refers to the effect that the transmission of one symbol or group of symbols has on the transmission of the following symbol or groups of bits. High gain amplifiers introduce AM/AM (amplitude modulation) and AM/PM (phase modulation) distortion as a result of the non-constant envelope nature of the signals that are provided as inputs to the amplifier. Because the data is digitally encoded on a waveform, the pulse shape of the waveform creates artifact portions, where preceding pulses combine to interfere with the particular pulse being sampled. This is known as intersymbol interference (ISI), and requires that the signal pulses be shaped to reduce the memory of the amplifier.
Multiple possible transmission paths of a signal through a transmitter exist for an input signal. A typical input signal into a HPA, such as a TWTA, undergoes a filtering effect by the transmitter hardware before the amplifier. The input signal also experiences filtering effects of the HPA as a result of its memory. Because the amplifier has memory, a symbol can follow different paths, depending on what symbols were transmitted before the current symbol period. The non-linearity of the amplifier distorts the filtered input signal due to its nonconstant envelope. By applying memory predistortion techniques, the ISI of the amplifier can be reduced, thus limiting the distortion.
Locally-adapted linear predistorters typically intend to invert the filtering prior to the HPA (pre-HPA filtering) and ignore filtering after the HPA (post-HPA filtering). The presence of filtering after the non-linearity of HPA will provide a linear signal at the receiver. The receiver equalizer typically suitably removes most filtering with minimal distortion so long as only linear distortion exists. However, post-HPA filtering complicates the linear predistortion by introducing non-linear, memory components to the signal. This non-linear memory interferes with the desired operation of the predistorter algorithm, thus, it is desirable to eliminate the post-HPA filtering to enhance the predistortion.